The present invention relates generally to methods of removing selenium (Se) from sulfur (S) recovered from leaching of residues of sulfide concentrates. It relates in particular to a liquid-liquid extraction process wherein selenium is transferred from an organic solution of selenium-contaminated sulfur into an aqueous solution which is subsequently separated from the organic solution.
Selenium content in sulfur recovered from leaching of residues of sulfide concentrates is usually too high for the sulfur to be marketable. To a lesser extent, the recovered sulfur may contain tellurium in quantities too high for the sulfur to be marketable.
Selenium and sulfur have very similar chemical properties, and atoms in selenium and sulfur molecules can substitute for each other. Binary compounds of selenium with sulfur have been known for more than one-hundred years. During the last two decades, it has been shown that selenium and sulfur can form diatomic molecules, chains of various lengths (Se.sub.x S.sub.y), or rings of various sizes (Se.sub.m S.sub.n where m+n=6, 7, 8 or 12) (E. Fluck, Gmelin Handbuch Der Anorganischen Chemie, Vol B2, Springer-Verlag, Berlin, 1984, pp. 280-315). Because of this, separation of selenium and sulfur, especially purification of sulfur which contains only a minor amount of selenium, can be difficult and expensive.
Selenium and sulfur have different boiling points, 445 and 685 degrees Centigrade (.degree. C.) respectively. While the difference in the boiling points is significant, in mixtures of molten selenium and sulfur, volatility of selenium is higher, and volatility of sulfur is lower than predicted by Raoult's law. This creates difficulty in separation by distillation. In addition, diatomic Se--S compounds have been found in gaseous phases evaporating from molten mixtures of selenium and sulfur at relatively high temperatures (E. Fluck, Gmelin Handbuch Der Anorganischen Chemie, Vol B2, Springer-Verlag, Berlin, 1984, pp. 280-315). This creates further difficulty in separation by distillation. However, as strength of S--Se bonding decreases with increasing temperature, separation of selenium and sulfur by distillation is still possible using tall distillation columns with a large number of distillation plates at relatively high temperature. It has been reported that, at Port Colborne Nickel Refinery, a twenty-seven meter (27 m) tall distillation column having 60 distillation plates was used to evaporate sulfur from anode slimes resulting from electrolysis of nickel matte (D. M. Chizhikov and V. P. Shchastlivyi, Selenium and Selenides, Collet's Ltd., London, 1968, pp. 104-107). The selenium in the sulfur was reduced from an initial 0.15 percent (%) by weight to five parts per million (5 ppm), with the evaporation conducted at 500.degree. C.
Chemical additives have been used to enhance separation of selenium from sulfur. Kuwano, in Japanese Patent 6,921,407, discloses an applied zone refining technique with additions of silver sulfide (Ag.sub.2 S) to separate selenium impurity from sulfur containing 200 ppm selenium. After a consolidation rate of two centimeters per second (2 cm/s) through a continuous zone refining furnace for fifty passes, 10, 40, 60 and 70% of selenium was removed with additions of 5, 10, 20 and 40% of Ag.sub.2 S respectively. In the aforementioned Japanese Patent, Kuwano also discloses evaporation of sulfur with addition of copper (Cu) to form copper selenide (CuSe), thereby lowering partial pressure of selenium in the gaseous phase. Distillation separation of selenium from sulfur using addition of silver (Ag) has been reported (H, Suzuki et al. Bulletin of the Chemical Society of Japan, Vol. 47 (R74), No. 3, pp. 757-758). Mixing Ag powder with sulfur containing 130 ppm of selenium, the evaporation was conducted at 600.degree.-700.degree. C. Distilled sulfur samples contained 117 ppm without addition of Ag, and 2.5 ppm of selenium with addition of 0.86 grams (g) of Ag per 1.0 g of sulfur.
In Japanese Patent 7,134,490, Nakane discloses removal of selenium from molten sulfur by chlorination of the sulfur with chlorine (Cl.sub.2) or sulfur chloride (S.sub.2 Cl.sub.2). Selenium present in the molten sulfur was converted to chlorides and then carried away by blowing air through the melt for about one-hour. Any chlorides remaining in the melt were removed by activated carbon, alumina (Al.sub.2 O.sub.3) or acid clay. The method was effective in removing about 50% of selenium impurity. Removal of 85% of selenium impurity by treating molten sulfur with magnesium chloride (MgCl.sub.2) and nitric acid (HNO.sub.3) has also been reported (M. N. slepanov et al., Journals of Chemical Industry (U.S.S.R), Vol. 18 (1941), No. 20, pp. 4-7).
Those skilled in the art to which the present invention pertains will recognize that while all of the above describes processes for removing selenium from sulfur have been successful to some degree, all involve one or more of large complex apparatus, high energy consumption, and a requirement for containing toxic or potentially toxic gasses. Clearly there is a need for a simpler more energy efficient process for removing selenium from sulfur.